Mid-IR type-II W and interband cascade lasers emitting high CW powers

Abstract

The I-V characteristics, lasing thresholds, and wallplug efficiencies of type-II "W" mid-IR diode lasers from 16 different wafers were studied in order to determine the influence of various device parameters. At T = 90 K, the wallplug efficiency for a 1-mm-long gain-guided device was > 10% and the slope efficiency was 142 mW/A (38% external quantum efficiency). When a 22-µm-wide ridge was lithographically defined on a 5-period "W" laser with a p-GaSb etch stop layer, the maximum cw operating temperature increased to 230 K. We also investigated 5-stage and 10-stage interband cascade lasers containing "W" active quantum wells. For 10-stage devices, the lowtemperature threshold current densities were somewhat higher than in the "W" diodes while at higher temperatures they were slightly lower. The threshold voltage was only ≈ 0.1 V larger than the photon energy multiplied by the number of stages, corresponding to a voltage efficiency of > 96%, while the differential series resistance-area product above threshold was as low as 0.21 mΩ·cm 2 at 100 K. At T = 78 K, the cw slope efficiency was 0.48 mW/A (126% external quantum efficiency), and a maximum cw power of 514 mW was produced by an epi-side-upmounted 2-mm-long 10-stage laser cavity with uncoated facets. A 5-stage 2-mm-long interband cascade laser produced ≈ 700 mW of output power at 80 K, with a maximum wallplug efficiency of 20% per facet.Keywords: diode laser, interband cascade laser, quantum well, mid-infrared. INTRODUCTIONAmbient-temperature cw operation remains unrealized for any electrically-pumped semiconductor laser in the 3.2-3.9 µm spectral region, in spite of the significant progress made by type-I 1 and quantum cascade 2,3 lasers on the short and long wavelength sides, respectively. An important contender to fill the gap is the antimonide type-II "W" structure, 4,5 which consists of a Ga(In)Sb hole quantum well (QW) surrounded by two InAs electron QWs, which are in turn bounded by AlGa(As)Sb barrier layers. Previous "W" diodes demonstrated pulsed lasing at room temperature 6,7 and cw operation to 218 K, 8,9 while interband cascade lasers (ICLs) with 12-25 stages of the "W" active region recently lased cw up to 214 K (λ = 3.4 µm) 10 and 237 K (λ = 3.2 µm). 11The purpose of the work reported in this article is twofold. First of all, a systematic study of "W" diode lasers with designs modified in a variety of ways to probe effects on the electrical and lasing characteristics was conducted.Since the antimonide material system remains relatively immature by comparison with GaAs-and InP-based structures, it has been necessary to address numerous growth and processing issues along the way to improving the laser performance. Most of the present structures were designed for low-temperature emission at λ ≈ 3.2 µm, although a few longer-wavelength devices (λ ≈ 3.8 µm and 4.4 µm) were also fabricated. Both broad gain-guided and narrow ridge geometries were investigated. * vurgaftman@nrl.navy.mil; phone 202-404-8604; fax 202-404-7530